I. Field of the Invention
This invention relates generally to the field of seismic prospecting and more particularly to a method and apparatus for injecting an acoustic pulse into the earth.
II. Description of the Prior Art
U.S. Pat. No. 4,284,165 issued Aug. 18, 1981 entitled "Acoustic Pulse Generator" describes apparatus incorporating means for driving an impact piston downwardly by compressed air to strike an earth contacting target. In accordance with this patent, compressed gas is provided by high volume accumulators interconnected with large area ports which penetrate the side wall of the housing in which the impact piston is slideably operated. The piston wall is upwardly tapered and presses against seals mounted on the correspondingly tapered housing bore above and below the entry ports. In this way, a constant breakaway force is exerted on the tapered piston wall through the ports. In operation, when the piston is released, compressed gas enters the housing bore through the ports when the top of the piston drops away from its upper seal means.
U.S. Pat. No. 4,284,164 issued Aug. 18, 1981 entitled "Acoustic Pulse Generator", also describes related apparatus incorporating means for driving a piston downwardly by compressed air to strike an earth-contacting target. As distinguished from U.S. Pat. No. 4,284,165, the piston in this patent has a side wall of constant diameter so that no initial downward breakaway force is exerted on the piston. The construction disclosed, however, also employs a side-ported housing for admission of air pressure, which reaches the top piston surface only after piston release occurs.
The invention to be described hereafter represents an improvement on the method and apparatus of the above prior art patent which is more particularly concerned with the means for efficient exploitation of compressed gas as a driving force in an impact device of this character.
It is therefore a general object of the present invention to provide an acoustic pulse generator of the type described with improved mass acceleration means.
It is a more particular object of the present invention to provide improved means for applying pressurized gas as an energy source in an acoustic pulse generator of the type described.
It is a still more specific object of the present invention to maximize the speed of operation of an acoustic pulse generator of the type described.
Other and further objects and advantages of the invention will become apparent from a consideration of the detailed description and drawings to follow.